Holographic device for formation of colored, polarized and angularly separated light beams and video image projector which utilizes it

ABSTRACT

A holographic device, which has first and second roughly coplanar and superposed holograms, which are illuminated by the same non-polarized light beam with at least first and second spectral compositions, the holograms each being recorded in a manner such that in its middle, the axis of the diffracted beam is perpendicular to the direction of the incident beam in this medium, the diffracted beams forming first and second angularly separated light beam with the same polarization, and having different spectral compositions.

The present invention relates to a holographic device for formation ofat least first and second colored, angularly separated light beams, andmore particularly, to a device for formation of such beams with the sameplanar polarization and having first and second predetermined spectralcompositions respectively. Still more particularly, the inventionrelates to a video image projector incorporating such a device.

A device is known from the patent U.S. Pat. No. 5,161,042, which isbased on the use of dichroic mirrors for angularly separating threelight beams of different colors illuminating a matrix screen of liquidcrystal cells through a grid of micro-lenses, for the projection of avideo image displayed on this screen. The three beams are separatedangularly in the same plane and are therefore suitable if the threeliquid crystal cells which define a pixel of the image registered on thescreen are aligned. They are not suitable if they are arranged at theapexes of a triangle, according to the configuration called “Δ”, forexample. Furthermore, the three beams obtained are not polarizedlinearly, and it is therefore not possible to do without the two crossedpolarizers which ordinarily equip a liquid crystal matrix screen andwhich have the disadvantage of absorbing a substantial quantity of theluminous energy.

Also known from the international patent application WO-A-92/09915 is aholographic device for illumination of such a screen by suitably coloredand polarized light beams, formed by three distinct sets of prisms andholograms, which make the device bulky. The present invention aims toproduce a holographic device for formation of colored and polarizedlight beams which are angularly separated, for the projection of a videoimage displayed on a matrix screen of liquid crystal cells, whose spacerequirement is as small as possible.

The present invention also aims to produce such a device making itpossible to angularly separate three such beams according to threenon-coplanar directions, for the illumination of a liquid crystal screenin which the three cells defining a pixel of the image are not arrangedin a line.

The present invention furthermore aims to produce such a device whichhas a high luminous yield and good compactness.

These aims of the invention are achieved, as are others which willappear upon reading of the following description, with a holographicdevice for formation of at least first and second angularly separatedlight beams, with the same planar polarization, and having first andsecond predetermined spectral compositions respectively, this devicebeing remarkable in that it has first and second roughly coplanar andsuperposed holograms, which are illuminated by the same nonpolarizedlight beam with at least said first and second spectral compositions,said holograms each being recorded in a manner such that in its middle,the axis of the diffracted beam is perpendicular to the direction of theincident beam, said diffracted beams forming said first and secondangularly separated light beams with the same polarization, and havingfirst and second predetermined spectral compositions respectively.

As will be seen in detail subsequently, the light beams thus formed areproduced by the same optical assembly, with a reduced space requirement.

According to one embodiment of the invention, the holograms are squeezedbetween two attached adjacent prisms, made of optical material. They arerecorded in a material with a refractive index roughly equal to that ofthe material constituting said prisms. They are illuminated by a beam oflight with a 45° incidence, the axes of the beams diffracted by theseholograms converging on the axis of the incident beam hitting theholograms and being in a plane perpendicular to this axis.

According to a first variant, the device includes a third hologramroughly coplanar with the other two, recorded so as to form a diffractedbeam of light with planar polarization with a third predeterminedspectral composition, whose axis is coplanar and converging with thoseof the other two diffracted beams. As will be seen subsequently, such adevice is suitable for illuminating matrix screens of liquid crystalcells in which the cells of the triplets, each of which defines a pixelof the displayed image, are aligned.

According to another variant of the device according to the invention, athird hologram is included which is not coplanar with the other two,recorded in a material with a different index from the index of theother two, and arranged to form a third diffracted beam whose axis isnot coplanar with the axes of the beams diffracted by the first andsecond holograms, said third beam being polarized linearly like theother two and having a third predetermined spectral composition. In thisvariant, the device includes a third cuneiform prism inserted betweenthe other two prisms, the first and second holograms being squeezedbetween a face of this third prism and a face of one of the otherprisms, while the third hologram is squeezed between another face ofthis third prism and a face of the other of the other prisms, the planesof the holograms converging on an edge common to the three prisms. Aswill be seen subsequently, this variant is suitable for the illuminationof a matrix screen of liquid crystal cells in which the cells of thetriplets of cells, each of which defined a pixel of the image to bedisplayed, are not aligned and are arranged, for example, in Δconfiguration

Other characteristics and advantages of the device according to thepresent invention will appear upon reading of the following descriptionand upon examination of the appended drawing in which:

FIGS. 1 and 2 diagrammatically represent the first and second variantsof the device according to the invention mentioned above, respectively,and

FIG. 3 is a diagram of the video image projector incorporating thedevice of FIG. 2.

Reference is made to FIG. 1 of the appended drawing in which the devicerepresented includes two identical rectangular prisms 1, 2, isosceles incross section perpendicular to a hypotenuse face, produced from atransparent optical material such as glass. These two prisms areattached according to their hypotenuse faces by squeezing between themat least first and second holograms 3 and 4 respectively, which arerecorded in at least one light-sensitive medium.

According to the invention, one of the two holograms, hologram 3, forexample, can be recorded in the medium by the interaction of two lightbeams with a predetermined spectral composition, in a manner such thatthese two beams interact in the material of the recording support,according to wave vectors oriented at 90° with respect to one another,in the plane of FIG. 1, for example. It is known that, upon reading of ahologram thus recorded, if the hologram is illuminated with a beam withsaid spectral composition, the wave vector of the diffracted beam isalso oriented at 90° with respect to the incident beam.

It is also known that the diffraction yield for the radiation componentpolarized in the plane of incidence (component p) is then zero whilethis yield for the component polarized perpendicularly to the plane ofincidence (component s) is 100%. For more details on thischaracteristic, it will be possible to consult the article of Kogelnikentitled “Waves in thick holograms”, published in The Bell SystemsTechnical Journal, 1969, pages 2909 to 2917.

The process of recording described above, although theoretically usablefor obtaining the holograms to be installed in the device according tothe present invention, is not that which is preferred in practice. Forreasons connected with the spectral sensitivities of the currentlyavailable hologram recording materials, it is preferable to use amonochromatic radiation source suited to the sensitivity of thisrecording material. In this case, the angle of the beams which interferewith the recording to create the spatially modulated distribution of therefractive index which constitutes the hologram is not necessarily 90°and can be calculated conventionally so that upon reading, one finds anangle of 90° in the hologram between the incident beam and thediffracted beam.

When hologram 3, obtained by either of the recording processes describedabove, is illuminated with a light beam with axis A, oriented at 45°from the normal to the hologram, having the predetermined spectralcomposition mentioned above, axis B of the diffracted beam is orientedin the plane of FIG. 1, perpendicular to axis A of the incident beam if,of course, the material in which the hologram has been recorded has anidentical refractive index or a refractive index which is at least veryclose in practice to the refractive index of the glass of prism 2. As anexample of such a material, it is possible to mention thelight-sensitive polymer marketed by the company Dupont de Nemours underthe reference HRF-100, which has a refractive index of 1.51.

Furthermore, in accordance with the instructions of the aforementionedKogelnik article, beam B is entirely polarized linearly, since the yieldof diffraction of component s of the light of beam A can be close to100%, while component p of this beam is absent from diffracted beam Band is found in the zero order beam with axis A′ colinear with axis A.

This characteristic is very advantageous in that in the application tothe projection of video images displayed on a matrix screen of liquidcrystal cells, it makes it possible to rid this screen of the polarizingfilm with which such a screen is ordinarily equipped, and which absorbsa large fraction of the light passing through it. The brightness of theprojected images is thus considerably improved with a projector such asthat represented in FIG. 3, which will be described in detailsubsequently.

Thus, the invention makes it possible to form a beam of light B which isentirely polarized linearly and with a predetermined spectralcomposition, and with a wave vector in the plane of FIG. 1, suitable forbeing used directly for the illumination of a matrix screen of liquidcrystal cells attached to a grid of micro-lenses for focusing of thelight of this beam on the cells of the screen, these cells controllingthe transmission of beams of light with said spectral compositiontowards a projection surface.

In practice, however, the projection of video images in color requiresthe use of at least two such light beams, and preferably three, forexample, beams of red, green and blue light focused by the grid ofmicro-lenses on the corresponding liquid crystal cells of triplets ofsuch cells each associated with a pixel of the image to be projected.These cells control the transmission of beams of red, green and bluelight towards the projection surface in order to cause an image toappear there, an image which is generally enlarged, as will be seenfurther on in connection with FIG. 3.

For this purpose, between the two prisms 1 and 2, besides hologram 3,hologram 4 and a third hologram (not represented in FIG. 1) arearranged, these three holograms being recorded, for example, in the samerecording material, by one of the processes described above.

Upon reading of these three holograms, the wave vectors of thediffracted beams must be oriented differently in a plane perpendicularto the plane of FIG. 1 and passing through axis B of the beam diffractedby hologram 3. For this purpose, the beams which interfere with therecording of hologram 4, for example, can be such that the wave vectorof one of the beams is oriented according to direction A and the wavevector of the other is inclined with respect to the plane of the figureand is in a plane perpendicular to this figure and passing through theaxis of beam B. Thus an angle of 90° is maintained between the beamswhich interact in the material of the hologram.

When the device of FIG. 1 is illuminated according to axis A with asingle nonpolarized light, covering the two spectral compositions usedfor the recording of holograms 3 and 4, they respectively diffract beamB as described above and beam C with the same planar polarization butwith a different spectral composition from that of beam B, according totwo directions inclined with respect to one another. Thus, the axis ofbeam C is inclined with respect to the plane of the figure while beingthe same in projection in the plane with the axis of beam B.

It can be seen that by arranging a third hologram between prisms 1 and 2and by recording it using two light beams with a third spectralcomposition in a manner such that the wave vector of one of the beams isoriented according to A and the wave vector of the other is inclinedwith respect to the axes of beams B and C in the plane perpendicular toFIG. 1 and passing through axes B and C, by diffraction of a singlelight beam with the three spectral compositions used to record theholograms, three diffracted beams with the same planar polarization butwith three different colors are formed, whose axes are coplanar andseparated angularly from one another. One of them is in the plane ofFIG. 1 and the other two are, for example, arranged symmetrically withrespect to this plane. In the application more particularly intended bythe present invention to the projection of video images displayed on amatrix screen of liquid crystal cells, a grid of micro-lenses takes thethree beams and focuses corresponding beams of light on cells of thescreen associated with the red, green and blue components, respectively,of the image to be projected.

The device described above has the advantage of a minimal spacerequirement, that of a single cube of glass constituted by theassemblage of prisms 1 and 2. However, it does not make it possible toilluminate a screen constituted by triplets of cells for red, green andblue colors respectively, when the cells of the triplet are not arrangedin a line, for example, in Δ configuration. FIG. 2 of the appendeddrawing diagrams a second variant of the device according to theinvention allowing this result to be attained.

In this figure, it is shown that the device, like the device of FIG. 1,includes rectangular prism 1 supporting, on its hypotenuse face,holograms 3 and 4 recorded in the same medium, for example, which playthe same part as those of the device of FIG. 1. A medium containing athird hologram 5 is placed between rectangular prism 2′ and cuneiformprism 6 of which one face is adjacent to holograms 3 and 4 and of whichanother face is adjacent to hologram 5. The planes of holograms 3 and 4,on one hand, and 5, on the other hand, converge on edge 7 common to thetree prisms 1, 2′ and 6. The angle at the summit a_(k) of the cuneiformprism 6, measured on this edge 7, has a value of which the determinationwill be explained subsequently. The cuneiform prism 6 and prism 2′together have a volume identical to that of prism 2 of the device ofFIG. 1. Thus the devices of FIGS. 1 and 2 both have the compact form ofa cube.

According to one characteristic of the device of FIG. 2, the mediumcontaining hologram 5 has a different refractive index from that ofholograms 3 and 4. For this purpose, it will be possible to use, forholograms 3 and 4, the aforementioned light-sensitive polymer of Dupont,and for the recording of hologram 5, a layer of bichromated gelatin withindex n₅=1.38. The value of this arrangement is explained below.

Hologram 5 is recorded by bringing about the interaction of two beams ofradiation with a predetermined spectral composition, different from thatused for holograms 3 and 4. Furthermore, the beams used for therecording of hologram 5 must interact at 90° with respect to one anotherin order to ensure the complete polarization of the diffracted beam. Ifone were to use, for hologram 5, during its recording with a referencebeam with orientation A, a material with an index equal to that of thematerial used for the recording of holograms 3 and 4, identical to thatof prism 2′, this condition of interaction would result in the formationof a diffracted beam with orientation D′ perpendicular to the directionA and therefore parallel to the plane containing the axes of beams B andC. Now, it is necessary for the illumination of the triplets of cellsarranged at the summits of a triangle in Δ configuration, for the axis Dof the beam diffracted by hologram 5 to be inclined with respect to theplane containing the axes of the beams B and C, so that the samemicro-lens can focus three beams of light which are taken from thesethree beams and which are crossed over its opening, on the three cellsin Δ configuration of the same triplet of cells.

Then, in order to regain, with the device of FIG. 2, the condition ofthe orthogonal nature of the directions of propagation in the medium ofhologram 5 of the incident and diffracted beams necessary for obtaininga high level of polarization of the diffracted beam and the angle ofinclination a=2a_(k), in prism 2′, of this beam with respect to theplane containing the axes of beams B and C, it is demonstrated that theangle at the summit a_(k) of cuneiform prism 6 must be connected withthe refractive indexes of the materials of the holograms by theequation:${\alpha_{k}} = {{{\arcsin \left( {{\frac{n_{5}}{n} \cdot \sin}\quad 45{^\circ}} \right)} - {45{^\circ}}}}$

in which n₅, n are the refractive indexes of the material of hologram 5,and of the material of holograms 3 and 4, respectively.

It is seen that the device of FIG. 2, in which the axes of beams B, Cand D form the edges of a triangular pyramid, makes it possible toilluminate, through a grid of micro-lenses for focusing which is knownin itself, all the liquid crystal cells of the matrix screen, in whichthe three cells which define a pixel of the image are arranged in Δconfiguration. This result is reached with a compact device, making itpossible to eliminate the polarizing film normally associated with sucha screen, and therefore the losses of luminous energy due to theabsorption of light in this film, in accordance with all the aims of thepresent invention.

The luminance of the image projected by a video projector equipped withthe device according to the invention is advantageously increased by theelimination of the polarizing film. Such a projector is described inconnection with the examination of FIG. 3 which diagrams, in the form ofa perspective view, the optical system of a projector of video imagesdisplayed on screen 10 of liquid crystal cells. The system includespolychromatic light source 11, for example, a source of natural whitelight which can be broken down conventionally into two components p ands with planar polarization, which are perpendicular to one another. Theoptical system also includes device 12 such as that represented in FIG.2, provided with holograms 13, 14, 15 recorded and arranged in themanner of holograms 3, 4 and 5 of the device of FIG. 2, respectively.The light from source 11, diffracted by holograms 13, 14, and 15,illuminates screen 10 through a grid of micro-lenses 16. Each of thelenses of this grid then focuses three beams of light 17 _(R), 17 _(V),17 _(B), red, green, and blue, respectively, for example, on threeliquid crystal cells R, V, and B, respectively, of the screen, definingtogether a pixel of the image displayed on screen 10, as represented forsurface element 10 a of screen 10.

These three light beams being entirely polarized linearly on coming outof device 12, screen 10 is free of the polarizing film with which it isconventionally equipped, which therefore reinforces the intensity of thelight which illuminates cells R, V, B as was seen above. These cells,associated with analyzer 10′, conventionally act as light valves,controlled in all-or-nothing [mode] by an electronic [device] which isknown in itself, an objective 18 taking the light transmitted by screen10 in order to project it on a projection surface (not represented) inwhich an enlarged image of that displayed on screen 10 appears.

In FIG. 3, device 12 only illuminates half of screen 10, the other halfbeing illuminated by identical device 12′ attached to device 12 andpivoted, around a horizontal axis, one quarter of a turn with respect tothe latter. Device 12′ receives, coming in, the zero order beam whichpasses through holograms 13, 14, and 15 of device 12, parallel to theaxis of the light beam coming from source 11. This zero order beam ismainly made up of component p of the light emitted by this source, sincethe majority of component s goes in the beams diffracted by holograms13, 14, and 15 as seen above. For device 12′ to receive mainly components, the only one diffracted in this device, half wave plate 19 isarranged coming out of device 12. A beam mainly containing component p,by passing through this plate, is transformed into a beam mainlycontaining component s, directed towards two return mirrors 20, 20′which shift this beam towards device 12′.

One sees that due to the use of the two devices 12, 12′, the light flowfrom source 11 is completely used, including the energy contained in thezero order beam of holograms 13, 14, 15. This is not possible with adevice for illumination of a matrix screen of liquid crystal cellsequipped with a conventional polarizer, which entirely absorbs one ofthe components with planar polarization of the incident beam, and whichcauses losses in the other usable component.

It is clear that if the triplets of cells R, V, B of screen 10 werealigned, devices of the type of FIG. 1 rather than of the type of FIG. 3would be installed in the optical system of FIG. 3.

It now appears that the invention indeed makes it possible to attain thestated aims, namely to produce a projector of video images with acompact optical system and with a high luminous yield, which can beadapted to matrix screens of liquid crystal cells with triplets whichare aligned or in Δ configuration.

What is claimed is:
 1. A holographic device for formation of at leastfirst and second angularly separated light beams, with the same planarpolarization, and having first and second predetermined spectralcompositions, respectively, said device comprising first and secondroughly coplanar and superposed holograms, which are illuminated by thesame nonpolarized incident light beam with at least said first andsecond spectral compositions, said holograms each being recorded in amedium in a manner such that in its middle, each hologram produces adiffracted beam the axis of which is perpendicular to the direction ofsaid nonpolarized incident beam in the medium, said diffracted beamsforming said first and second angularly separated light beams with thesame polarization, and having the first and second predeterminedspectral compositions, respectively.
 2. Device according to claim 1,wherein said holograms are placed between two attached adjacent prismsmade of optical material.
 3. Device according to claim 2, wherein saidholograms are recorded in a material with a refractive index (n) roughlyequal to that of the material constituting said prisms.
 4. Deviceaccording to claim 3, wherein said holograms are illuminated at a 45°incidence, and the axes of the beams diffracted by the hologramsconverge on the axis of said nonpolarized incident light beam and are ina plane perpendicular to that axis.
 5. Device according to claim 1,wherein the device comprises a third hologram roughly coplanar with theother two, recorded so as to form a diffracted beam of light with planarpolarization with a third predetermined spectral composition, whose axisis coplanar and converging with those of the other two diffracted beams.6. Device according to claim 1, wherein (a) the first and secondholograms are recorded in a material having an index of refraction (n)and (b) the device comprises a third hologram which is not coplanar withthe other two holograms, is recorded in a material with a differentindex from the index of the other two, and is arranged in order to forma third diffracted beam whose axis is not coplanar with the axes of thebeams diffracted by first and second holograms and perpendicular, in themedium of said third hologram, to the axis of said nonpolarized incidentlight beam, said third beam being polarized linearly like the other twoand having a third predetermined spectral composition.
 7. Deviceaccording to claim 6, wherein the device comprises a third cuneiformprism inserted between two other prisms, said first and second hologramsbeing squeezed between a face of the third prism and a face of one ofthe other prisms, and the third hologram being squeezed between anotherface of the third prism and a face of the other of the other two prisms,the planes of the holograms converging on an edge common to the threeprisms.
 8. Device according to claim 7, wherein the angle (α) ofinclination of the third diffracted beam with respect to the plane ofthe axes of the other two diffracted beams is connected with the angleat the apex α_(k) of the cuneiform prism on the edge common to the threeprisms and with the refractive indexes of the materials of the hologramsby the equation:${\underset{\_}{\alpha }\left\lbrack {\alpha_{k}} \right\rbrack} = {{{2\quad \alpha_{k}}} = {2{{{{arc}\quad {\sin \left( {{\frac{n_{5}}{n} \cdot \sin}\quad 45{^\circ}} \right)}} - {45{^\circ}}}}}}$

in which n is the refractive index of the material of the first andsecond holograms, and n₅ the refractive index of the material of thethird hologram.
 9. A projector of video images comprising: (a) a firstassembly which comprises a matrix screen of liquid crystal cellsattached to a grid of optical micro-lenses arranged for focusing atleast two beams of polarized light with predetermined spectralcompositions on at least two corresponding cells of the screen, and (b)a second assembly which comprises at least one holographic deviceaccording to claim 1 for illuminating the micro-lenses of the grid withat least two beams of light with said spectral compositions. 10.Projector according to claim 9, wherein the second assembly comprisesfirst and second holographic devices, each of which illuminates aparticular zone of the grid of micro-lenses, one of said holographicdevices being illuminated directly by a light source and the other ofsaid holographic devices collecting the zero order beam from theholograms of the first holographic device.
 11. Projector according toclaim 10, wherein the second assembly comprises a half wave plate and apair of mirrors arranged on the zero order beam of the first holographicdevice, in order to rotate the plane of polarization of said zero orderbeam and to shift its axis in the direction of that of the secondholographic device.
 12. A projector of video images comprising: (a) afirst assembly which comprises a matrix screen of liquid crystal cellsattached to a grid of optical micro-lenses arranged for focusing atleast two beams of polarized light with predetermined spectralcompositions on at least two corresponding cells of the screen, and (b)a second assembly which comprises at least one holographic deviceaccording to claim 5 for illuminating the micro-lenses of the grid withat least two beams of light with said spectral compositions. 13.Projector according to claim 12, wherein the second assembly comprisesfirst and second holographic devices, each of which illuminates aparticular zone of the grid of micro-lenses, one of said holographicdevices being illuminated directly by a light source and the other ofsaid holographic devices collecting the zero order beam from theholograms of the first holographic device.
 14. Projector according toclaim 13, wherein the second assembly comprises a half wave plate and apair of mirrors arranged on the zero order beam of the first holographicdevice, in order to rotate the plane of polarization of said zero orderbeam and to shift its axis in the direction of that of the secondholographic device.
 15. A projector of video images comprising: (a) afirst assembly comprising a matrix screen of liquid crystal cellsattached to a grid of optical micro-lenses arranged for focusing atleast two beams of polarized light with predetermined spectralcompositions on at least two corresponding cells of the screen, and (b)a second assembly comprising at least one holographic device accordingto claim 6 for illuminating the micro-lenses of the grid with at leasttwo beams of light with said spectral compositions.
 16. Projectoraccording to claim 15, wherein the second assembly comprises first andsecond holographic devices, each of which illuminates a particular zoneof the grid of micro-lenses, one of said holographic devices beingilluminated directly by a light source and the other of said holographicdevices collecting the zero order beam from the holograms of the firstholographic device.
 17. Projector according to claim 16, wherein thesecond assembly comprises a half wave plate and a pair of mirrorsarranged on the zero order beam of the first holographic device, inorder to rotate the plane of polarization of said zero order beam and toshift its axis in the direction of that of the second holographicdevice.
 18. A projector of video images comprising: (a) a first assemblywhich comprises a matrix screen of liquid crystal cells attached to agrid of optical micro-lenses arranged for focusing at least two beams ofpolarized light with predetermined spectral compositions on at least twocorresponding cells of the screen, and (b) a second assembly whichcomprises at least one holographic device according to claim 7 forilluminating the micro-lenses of the grid with at least two beams oflight with said spectral compositions.
 19. Projector according to claim15, wherein the second assembly comprises first and second holographicdevices, each of which illuminates a particular zone of the grid ofmicro-lenses, one of said holographic devices being illuminated directlyby a light source and the other of said holographic devices collectingthe zero order beam from the holograms of the first holographic device.20. Projector according to claim 19, wherein the second assemblycomprises a half wave plate and a pair of mirrors arranged on the zeroorder beam of the first holographic device, in order to rotate the planeof polarization of said zero order beam and to shift its axis in thedirection of that of the second holographic device.